rails/rails — pull request #18099 by robsonmarques (2014-12-19 18:04:12+00:00)
Title: Fix inaccurate docs in active_model errors

The default value for the argument `message` in `ActiveModel::Errors#add` has a new behavior
since ca99ab2481d44d67bc392d0ec1125ff1439e9f94.

```
Before
person.errors.add(:name, nil)
# => ["is invalid"]
After
person.errors.add(:name, nil)
# => [nil]
```